It is generally desirable to reduce power consumption of electronic devices having receiving functionalities. Especially battery-operated mobile terminals benefit from a reduced receiver power consumption. The benefits include longer stand-by and operational times.
Mobile terminals usually conform to one or more mobile communication standards that define, inter alia, operational states for the receiver. As an example, the Long Term Evolution (LTE) standard of the 3rd Generation Partnership Project (3GPP) specifies so-called “idle” and “connected” states for the physical layer of a mobile terminal (also referred to as User Equipment, or UE, in the LTE standard). The physical layer includes receiver components, the operation of which is influenced by the current state setting.
When the UE is in idle state, there are no ongoing transfers in the receiving and transmitting directions. The UE is only waking-up from time to time to check whether a connection request is coming in. An incoming connection request is signalled at so-called paging occasions. In idle state, the power consumption is therefore heavily reduced because the receiver components are switched off most of the time and only briefly switched on at paging occasions.
In connected state, the receiver components are switched on most of the time as the UE has to listen to the Physical Downlink Control Channel (PDCCH), which is transmitted in a first portion of a sub-frame. The PDCCH is used to transfer scheduling grants indicating that there will be a transmission on the Physical Downlink Shared Channel (PDSCH) to the UE in a subsequent second portion of the current sub-frame. In case a PDSCH transmission is indicated to the UE (either in the PDCCH or by semi-persistent scheduling), the remainder of the sub-frame has to be received and the PDSCH has to be decoded. Reception must also continue in other scenarios such as intra-frequency measurements or Broadcast Channel (BCH) readings.
There are still many scenarios in connected state in which decoding of the first sub-frame portion reveals that the remainder of the sub-frame following the PDCCH is of no interest to the UE and in which reception can be terminated until the next sub-frame arrives. Terminating reception by switching off one or more receiver components during the resulting short gap between the end of the first portion of one sub-frame and the beginning of the next sub-frame is also referred to as micro sleep.
In the LTE standard, a sub-frame has a duration of 1 ms and downlink transmissions are based on Orthogonal Frequency Division Multiplexing (OFDM). OFDM-based systems use block processing that includes a Fast Fourier Transform (FFT) for OFDM de-modulation. The digital receiver domain (Digital Front End, or DFE) before the FFT is based on sample processing. The PDCCH can be spread over up to 4 OFDM symbols for a system bandwidth of 1.4 MHz and over up to 3 OFDM symbols for larger bandwidths.
FIG. 1 shows a schematic timing diagram illustrating the processes of entering and leaving a micro sleep mode in an exemplary LTE scenario in which the PDCCH is spread over 3 OFDM symbols. In a regular reception mode (“Rx on period” in FIG. 1), a down-converted Radio Frequency (RF) signal from an analog radio front end is analog-to digital converted at a given sampling rate. The resulting signal samples are buffered in a memory for being subjected in blocks to FFT processing. The FFT processing results in a de-modulation of the received OFDM symbols, including the OFDM symbols that pertain to the PDCCH. After OFDM de-modulation, channel estimation and de-mapping steps are performed.
In a further step, the PDCCH is decoded to determine whether the remainder of the sub-frame has to be received also (and whether the PDSCH has to be decoded), or whether the receiver can enter a micro sleep mode (“Rx off period” in FIG. 1), in which one or more receiver components are switched off. The time it takes to enter the micro sleep mode (“‘Switching off’ period”) is typically rather short and therefore not illustrated in FIG. 1. On the other hand, the micro sleep mode has to be left early enough (“‘Switching on’ period” in FIG. 1) to ensure that the regular reception mode is entered again before the next sub-frame arrives.
In the micro sleep mode, it is desirable to entirely switch off the Radio Frequency (RF) part of the receiver. However, switching off the RF part also implies that the oscillator frequency and phase reference for down-conversion are lost (and need to be synchronized again when reception re-starts for the next sub-frame). Moreover, the sampling phase of the Analog-to-Digital Converter (ADC) is not necessarily maintained unchanged over the micro sleep gap. Hence, the phase of the received signal before and after the micro sleep gap will in general exhibit a discontinuity.
As a result of the phase discontinuity of the received signal, the corresponding equivalent baseband channel also exhibits a discontinuity. Therefore, channel estimation after the micro sleep gap cannot simply employ filtering of reference symbols (pilot symbols) before and after the micro sleep gap. Rather, channel estimation needs to start anew with filtering of only the reference symbols received after the micro sleep gap. As a consequence, channel estimation performance is degraded for the symbols (and sub-frames) immediately following a micro sleep gap.
The control channel (PDCCH) has to be received and decoded first after a micro sleep gap. However, due to the temporary degradation of the channel estimation performance after the micro sleep gap, the error rates for receiving control information (and possibly user data) substantially increase. To avoid the phase discontinuity and to thus combat the increased error rates, the oscillator with its Phase-Locked Loop (PLL) may remain switched on during in the micro sleep mode. However, such an approach significantly decreases the power savings in the micro sleep mode. In addition, many receiver configurations do not permit switching off only certain receiver components while maintaining frequency synchronization and keeping the PLL switched on.
It has been found that micro sleep concepts of the type discussed above help to reduce the power consumed by the receiver. It would nonetheless be desirable to achieve a significant reduction in receiver power consumption during the micro sleep mode while at the same time avoiding increased error rates immediately after the micro sleep mode has been left. In more general terms, it would be desirable to avoid the negative impact of the phase discontinuity on the channel estimation process.